Try to fast except if you are on hajj
Fasting on this day is an expiation for two years.
The Messenger of Allaah (peace and blessings of Allaah be upon him) was asked about fasting on the Day of ‘Arafah. He said, “It expiates for the sins of the previous year and of the coming year.” [Muslim]
Repent and do a lot of istighfaar.
It is the day of forgiveness of sins, freedom from the Fire and pride in the people who are there:
The Prophet (peace and blessings of Allaah be upon him) said: “There is no day on which Allaah frees more people from the Fire than the Day of ‘Arafah. He comes close and expresses His pride to the angels, saying, ‘What do these people want?’” [Muslim]
Do a lot of adhkaar and Supplications
The Prophet (blessings and peace of Allah be upon him) said: “The best of supplication is supplication of the day of Arafat, and the best of what I and the Prophets before me said is: Laa ilaaha illa Allah wahdahu, laa shareeka lah, lahu’l-mulk wa lahu’l-hamd wa huwa ‘ala kulli shay’in qadeer (There is no god but Allah alone, with no partner or associate; His is the Dominion, to Him be praise, and He has power over all things).”
May Allaah grant us all the benefits of this day.